FYI Gmail has SMTP/POP service, and has the added bonus of excellent virus/spam filtering and no rational size limits.
I would suggest a linux system with the main system being on a readonly partition. Then use unionfs/aufs to give them readwrite access. Worst case scenario you can blow away the RW partition and it will be restored to your defaults.
As for Window Manager, I'd give'm IceWM locked down with a Windows Style Start Menu. Put anything they can have access to there. (Maybe even a "Restore To Defaults" script.)
If you need remote access setup SSH to autostart on dialup connection, maybe with a No-IP client. If you set a decent password there is little risk for a mostly offline computer, and it would give you access. If ssh port is not an option you could setup some type of reverse ssh proxy connection.. but that might take up valuable bandwidth unless you give them a button they have to push to initiate the connection.